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Abstract:

Provided is a quantum encryption communication apparatus of a
transmission side which performs a communication process based on quantum
encryption, including: a light source unit which generates a light pulse;
a polarization modulating unit which performs polarization modulation of
the light pulse by using a variable wavelength plate; and a controller
which drives the variable wavelength plate to convert a polarization
state of the light pulse to one of a plurality of predetermined
polarization bases at random.

Claims:

1. A quantum encryption communication apparatus of a transmission side
which performs a communication process based on quantum encryption,
comprising: a light source unit which generates a light pulse; a
polarization modulating unit which performs polarization modulation of
the light pulse by using a variable wavelength plate; and a controller
which drives the variable wavelength plate to convert a polarization
state of the light pulse to one of a plurality of predetermined
polarization bases at random.

2. The quantum encryption communication apparatus according to claim 1,
wherein a liquid crystal retarder is used as the variable wavelength
plate.

3. The quantum encryption communication apparatus according to claim 2,
wherein a polarizer is integrally disposed in a light pulse incidence
surface side of the liquid crystal retarder.

4. The quantum encryption communication apparatus according to claim 3,
wherein a second liquid crystal retarder of which the optical axis is
tilted by 45 degrees with respect to an optical axis of the liquid
crystal retarder is disposed in an emitting surface side of the liquid
crystal retarder.

5. The quantum encryption communication apparatus according to claim 3,
wherein an intensity modulator is disposed between the polarizer and the
liquid crystal retarder.

6. The quantum encryption communication apparatus according to claim 5,
wherein the intensity modulator is configured with a liquid crystal
retarder and a polarizer.

7. A quantum encryption communication method of a quantum encryption
communication apparatus of a transmission side which performs a
communication process based on quantum encryption, comprising: allowing a
light source unit to generate a light pulse; performing polarization
modulation of the light pulse by using a variable wavelength plate;
allowing a controller to drive the variable wavelength plate to convert a
polarization state of the light pulse to one of a plurality of
predetermined polarization bases at random.

8. A quantum encryption communication system which performs a
communication process based on quantum encryption, wherein a quantum
encryption communication apparatus of a transmission side includes: a
light source unit which generates a light pulse; a polarization
modulating unit which performs polarization modulation of the light pulse
by using a variable wavelength plate and emits the polarization-modulated
light pulse to a communication line; and a controller which drives the
variable wavelength plate to convert a polarization state of the light
pulse to one of a plurality of predetermined polarization bases at
random, and wherein a quantum encryption communication apparatus of a
reception side includes: an optical unit which distributes the light
pulse emitted from the quantum encryption communication terminal of the
transmission side to each polarization basis; and a light-receiving unit
which detects the light pulse, which is distributed to each of the
polarization bases, with respect to each of the polarization bases.

Description:

BACKGROUND

[0001] The present disclosure relates to a quantum encryption
communication apparatus, a quantum encryption communication method, and a
quantum encryption communication system, and more particularly, to a
miniaturized quantum encryption communication apparatus performing
quantum encryption communication, which is capable of being mounted on a
portable electronic apparatus or the like.

[0002] In the related art, in a communication performed in the Internet or
the like, security has been kept by an encryption technique. The
encryption systems are mainly divided into two encryption systems of a
common key encryption system and a public key encryption system. For
example, at present, AES (Advanced Encryption Standard) or the like has
been widely used as the common key encryption system, and RSA or the like
has been widely used as the public key encryption system.

[0003] In the common key encryption system, two parties that perform
communication retain a common secret key. A transmitter encrypts a
plaintext by using the secret key to generate a ciphertext, and a
receiver decodes the ciphertext by using the same secret key to obtain
the original plaintext.

[0004] In the common key encryption system, the important factor in the
retaining of security is to keep the secret of the key. In the common key
encryption system, if so-called "brute-force attack" which searches for
the key by the brute-force is made, the key is revealed with a high
probability. In the common key encryption system which is used at
present, it is estimated that an impractically large amount of resources
(performance of computation or an amount of computation) for making the
brute-force attack would be necessary. Therefore, at present, it may be
considered that the common key encryption system is safe. However, in the
future, it may be predicted that the brute-force attack is a practical
attack due to improvement in the performance of computers or the like. In
fact, it is recommended that a method called 2-key TDES (Triple DES)
which has been used in the related art be replaced by the AES.

[0005] With respect to an attack including the brute-force attack, the
security is reinforced by using a method of frequently updating the
common key. In other words, although an attacker eavesdrops on the
communication and acquires a key, if the key is frequently updated, the
amount of the ciphertext which may be deciphered with the key is reduced,
so that the overall amount of the information which is acquired by the
attacker is relatively lowered.

[0006] As one of the methods of frequently updating the common key, as
disclosed in Japanese Patent No. 4015385, a method of performing quantum
key distribution (QKD: Quantum Key Distribution) by using quantum
encryption communication is proposed. The quantum key distribution is a
protocol which generates a common secret key between the two parties
which are connected by a communication line which may transmit quantum
state and a general communication line. The protocol is based on quantum
mechanical principles. Therefore, although an attacker eavesdrops on the
communication line, it is considered that the information on the
generated secret key is not leaked to the attacker. If the quantum key
distribution protocol is used, the secret key may be safely shared by the
two parties which are separated from each other, and the key may be
generated at any time by using the quantum key distribution protocol, so
that the aforementioned updating of the common key may be frequently
performed. In this manner, by combining the quantum key distribution and
the common key encryption, it is possible to reinforce the security of
the common key encryption system.

[0007] In the quantum key distribution, for example, the BB84 protocol or
the 6-state type protocol as an extension of the B84 protocol disclosed
in "Secret key can be obtained from both compatible and incompatible
measurements in the six-state QKD protocol" (Matsumoto Ryutaro, IEICE
Technical Report IT2007-43, ISEC20070140, WBS2007-74 (2008-02)) has been
used. In addition, as disclosed in Japanese Unexamined Patent Application
Publication No 2007-286551, a decoy method is used where the intensity
modulation of the light pulse is performed, so that it is possible to
further increase the encrypted intensity of the quantum key distribution.

SUMMARY

[0008] However, in the quantum encryption communication of the related
art, since a phase modulator or the like is used for the long-distance
communication using an optical cable, the communication apparatus is
configured to have a large size. For example, since the communication
apparatus has a size such that the communication apparatus is contained
in a rack, it is difficult to adapt the communication apparatus to a
portable electronic apparatus, for example, a mobile phone, a PDA, a
tablet type PC, an electronic book reader, a notebook type PC, or the
like.

[0009] It is desirable to provide a quantum encryption communication
apparatus capable of being embedded in a portable electronic apparatus or
the like and performing quantum encryption communication, a quantum
encryption communication method, and a quantum encryption communication
system.

[0010] According to a first embodiment of the present disclosure, there is
provided a quantum encryption communication apparatus of a transmission
side which performs a communication process based on quantum encryption,
including: a light source unit which generates a light pulse; a
polarization modulating unit which performs polarization modulation of
the light pulse by using a variable wavelength plate; and a controller
which drives the variable wavelength plate to convert a polarization
state of the light pulse to one of a plurality of predetermined
polarization bases at random.

[0011] In embodiment of the present disclosure, the light pulse is
generated by the light source unit which is configured by using, for
example, a semiconductor light-emitting device. In the polarization
modulating unit, the polarization modulation of the light pulse is
performed by using the liquid crystal retarder as the variable wavelength
plate. The controller drives the liquid crystal retarder to convert the
polarization state of the light pulse to one of a plurality of the
predetermined polarization bases. In this manner, the quantum encryption
communication apparatus of the transmission side outputs the
polarization-modulated light pulse. In addition, the polarizer is
integrally disposed in the light pulse incidence surface side of the
liquid crystal retarder. In addition, the second liquid crystal retarder
of which the optical axis is tilted by 45 degrees with respect to the
optical axis of the liquid crystal retarder is disposed in the emitting
surface side of the liquid crystal retarder, and the two liquid crystal
retarders are driven by the controller, so that the quantum encryption
communication using a 6-state type protocol is performed. In addition,
the intensity modulator, for example, the intensity modulator which is
configured with the liquid crystal retarder and the polarizer is disposed
between the polarizer and the liquid crystal retarder to perform the
conversion of the intensity of the light pulse, so that the quantum
encryption communication is performed.

[0012] According to a second embodiment of the present disclosure, there
is provided a quantum encryption communication method of a quantum
encryption communication apparatus of a transmission side which performs
a communication process based on quantum encryption, including: allowing
a light source unit to generate a light pulse; performing polarization
modulation of the light pulse by using a variable wavelength plate;
allowing a controller to drive the variable wavelength plate to convert a
polarization state of the light pulse to one of a plurality of
predetermined polarization bases at random.

[0013] According to a third embodiment of the present disclosure, there is
provided a quantum encryption communication system which performs a
communication process based on quantum encryption, wherein a quantum
encryption communication apparatus of a transmission side includes: a
light source unit which generates a light pulse; a polarization
modulating unit which performs polarization modulation of the light pulse
by using a variable wavelength plate and emits the polarization-modulated
light pulse to a communication line; a controller which drives the
variable wavelength plate to convert a polarization state of the light
pulse to one of a plurality of predetermined polarization bases at
random, and wherein a quantum encryption communication apparatus of a
reception side includes: an optical unit which distributes the light
pulse emitted from the quantum encryption communication terminal of the
transmission side to each polarization basis; and a light-receiving unit
which detects the light pulse, which is distributed to each of the
polarization bases, with respect to each of the polarization bases.

[0014] According to the present disclosure, it is possible to convert a
polarization state of a light pulse generated by a light source unit to
one of a plurality of predetermined polarization bases at random by using
a variable wavelength plate, for example, a liquid crystal retarder.
Therefore, since a polarization-modulated light pulse may be emitted from
a transmission side to a reception side by using a simple configuration,
it is possible to miniaturize a quantum encryption communication
apparatus or system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a diagram illustrating an overall configuration of a
quantum encryption communication system;

[0016]FIG. 2 is a diagram illustrating a configuration according to a
first embodiment of the present disclosure;

[0017]FIG. 3 is a diagram illustrating polarization modulation performed
by a liquid crystal retarder;

[0018]FIG. 4 is a diagram illustrating a configuration according to a
second embodiment of the present disclosure;

[0019]FIG. 5 is a diagram illustrating another configuration according to
the second embodiment of the present disclosure;

[0020]FIG. 6 is a diagram illustrating polarization modulation performed
by a liquid crystal retarder and a 1/4 wavelength plate;

[0021]FIG. 7 is a diagram illustrating a configuration according to a
third embodiment of the present disclosure;

[0022]FIG. 8 is a diagram illustrating polarization modulation performed
by liquid crystal retarders;

[0023]FIG. 9 is a diagram illustrating a configuration according to a
fourth embodiment of the present disclosure;

[0024]FIG. 10 is a diagram illustrating a configuration according to a
fifth embodiment of the present disclosure;

[0025]FIG. 11 is a diagram illustrating a state where a quantum
encryption communication apparatus is assembled into a mobile phone;

[0026]FIG. 12 is a diagram illustrating a state where a quantum
encryption communication apparatus is assembled into a notebook type PC;

[0027]FIG. 13 is a diagram illustrating a configuration in a state where
a quantum encryption communication apparatus is assembled into a notebook
type PC.

DETAILED DESCRIPTION OF EMBODIMENTS

[0028] Hereinafter, embodiments of the present disclosure will be
described. In addition, the description is made in the following order.

1. Overall Configuration and Operations of Quantum Encryption
Communication System

[0035]FIG. 1 is a diagram illustrating an overall configuration of a
quantum encryption communication system. The quantum encryption
communication system 10 includes a transmission side quantum encryption
communication apparatus (hereinafter, referred to as a "transmission side
communication apparatus") 20 and a reception side quantum encryption
communication apparatus (hereinafter, referred to as a "reception side
communication apparatus") 30. The transmission side communication
apparatus 20 and the reception side communication apparatus 30 are
connected to each other through a quantum communication line 51 and a
classical communication line 55.

[0037] The light source unit 21 is configured with a semiconductor
light-emitting device such as a laser diode or an LED, a lens which
collimates a light pulse emitted from the semiconductor light-emitting
device, and the like. The light emission of the light source unit 21 is
controlled by the controller 26.

[0038] The polarization modulating unit 22 converts the polarization state
of the light pulse emitted from the light source unit 21 to one of a
plurality of predetermined polarization bases. The polarization
modulating unit 22 is configured by using a variable wavelength plate,
for example, a liquid crystal retarder. The polarization modulating unit
22 performs polarization modulation based on the control signal of the
controller 26, converts the polarization state of the light pulse emitted
from the light source unit 21 to one of a plurality of the predetermined
polarization bases at a high speed based on the control signal, and emits
the light pulse through the quantum communication line 51 to the
reception side communication apparatus 30.

[0039] The key memory 23 stores the common key KYc generated by the
controller 26. In addition, the encrypting/decoding unit 24 encrypts a
communication text DVa using encryption or decodes an encrypted
communication text DVae by using the common key KYc stored in the key
memory 23.

[0040] The communication unit 25 transmits a communication text DVb which
is not encrypted or the communication text DVae encrypted by the
encrypting/decoding unit 24 through the classical communication line 55
to the reception side communication apparatus 30. In addition, the
communication unit 25 receives the communication text transmitted from
the reception side communication apparatus 30 through the classical
communication line 55. In the case where the received communication text
is not encrypted, the communication unit 25 supplies the received
communication text DVb to a signal processing unit (not shown). In
addition, in the case where the received communication text is encrypted,
the communication unit 25 supplies the received communication text DVae
to the encrypting/decoding unit 24. Therefore, the decoded communication
text DVa is supplied from the encrypting/decoding unit 24 to the signal
processing unit.

[0041] In order to perform the quantum encryption communication, the
controller 26 controls the emission of the light pulse from the light
source unit 21 or controls the polarization modulation which the
polarization modulating unit 22 performs on the emitted light pulse. In
addition, the controller 26 performs communication with the reception
side communication apparatus 30 through the communication unit 25 or the
classical communication line 55 and performs a process of generating the
common key from the communication result of the quantum encryption
communication, communication control of the communication text, control
of encrypting or decoding using the common key, or the like.

[0042] The reception side communication apparatus 30 includes an optical
unit 31, a light-receiving unit 32, a key memory 33, an
encrypting/decoding unit 34, a communication unit 35, and a controller
36.

[0043] The optical unit 31 distributes the polarization-modulated light
pulse, which is supplied from the transmission side communication
apparatus 20 through the quantum communication line 51, to the
polarization bases. The light-receiving unit 32 detects the light pulse,
which is distributed to each of the polarization bases, with respect to
each of the polarization bases and outputs the detection result to the
controller 36.

[0044] The key memory 33 stores the common key KYc which is generated
based on the detection result from the light-receiving unit 32 by the
controller 36. In addition, the encrypting/decoding unit 34 encrypts the
communication text DVa using encryption or decodes the encrypted
communication text DVae by using the common key KYc stored in the key
memory 33.

[0045] The communication unit 35 transmits the communication text DVb
which is not encrypted or the communication text DVae encrypted by the
encrypting/decoding unit 34 through the classical communication line 55
to the transmission side communication apparatus 20. In addition, the
communication unit 35 receives the communication text transmitted from
the transmission side communication apparatus 20 through the classical
communication line 55. In the case where Lire received communication text
is not encrypted, the communication unit 35 supplies the received
communication text DVb to a signal processing unit (not shown). In
addition, in the case where the received communication text is encrypted,
the communication unit 35 supplies the received communication text DVae
to the encrypting/decoding unit 34. Therefore, the decoded communication
text DVa is supplied from the encrypting/decoding unit 34 to the signal
processing unit.

[0046] The controller 36 performs communication with the transmission side
communication apparatus 20 through the communication unit 35 or the
classical communication line 55 by using the detection result of the
light-receiving unit 32 and performs a process of generating the common
key from the communication result of the quantum encryption
communication, communication control of the communication text, and
controls encrypting or decoding using the common key.

[0047] In the quantum encryption communication system having this
configuration, a random number is independently generated by the
controller 26 of the transmission side communication apparatus 20; the
generated random number is supplied to the polarization modulating unit
22; and the polarization modulation of the optical signal passing through
the quantum communication line 51 is performed. In addition, the
controller 36 of the reception side communication apparatus 30 generates
a receiving signal based on the light reception result of the
light-receiving unit 32, performs error correction, privacy
amplification, or the like on the receiving signal, and performs
generating or updating the common key which is common to the transmission
side communication apparatus 20 and the reception side communication
apparatus 30. In addition, the controller 36 stores the common key in the
key memories 23 and 33.

2. First Embodiment

[0048] In a first embodiment, the quantum key distribution is performed by
using a light pulse propagating through a free space according to the
BB84 protocol using the four types of polarization states including the
two types of linear polarizations and the two types of circular
polarizations.

2-1. Configuration of First Embodiment

[0049]FIG. 2 illustrates a configuration of the first embodiment of the
present disclosure. In addition, FIG. 2 illustrates the configuration of
the light source unit 21, the polarization modulating unit 22, the
optical unit 31, and the light-receiving unit 32 illustrated in FIG. 1.

[0050] The light source unit 21 of the transmission side communication
apparatus 20 is configured by using a semiconductor light-emitting device
211 such as a laser diode or an LED, a lens 212 which collimates a light
pulse emitted from the semiconductor light-emitting device 211.

[0051] A liquid crystal retarder 221 which converts the polarization state
of the collimated light pulse to one of the four types of polarization
states is used in the polarization modulating unit 22. The liquid crystal
retarder 221 is disposed so that the optical axis is tilted by 45 degrees
with respect to the direction of the linear polarization of the light
pulse emitted from the light source unit 21. The liquid crystal retarder
221 changes the phase difference occurring in the polarization components
along the FAST axis and the SLOW axis according to the control signal of
the controller 26.

[0052] In addition, in the polarization modulating unit 22, in the case
where the light emitted from the light source unit 21 is not the linear
polarization or the case where it is difficult to accurately control the
polarization direction with respect to the optical axis of the liquid
crystal retarder 221 although the light is the linear polarization, a
polarizer 225 is disposed in the light pulse incidence surface side of
the liquid crystal retarder 221. For example, the polarizer 225 is
integrated with the liquid crystal retarder 221 in the light pulse
incidence surface side of the liquid crystal retarder 221 so that the
optical axis of the liquid crystal retarder 221 is set to be tilted by 45
degrees with respect to the linear polarization emitted from the
polarizer 225. According to this configuration of the polarization
modulating unit 22, although the position of the polarization modulating
unit 22 is not accurately controlled with respect to the light source
unit 21, it is possible to set the polarization direction and the optical
axis of the liquid crystal retarder 221 at a desired angle.

[0053] The optical unit 31 of the reception side communication apparatus
30 includes a non-polarization beam splitter 311, polarization beam
splitters 312 and 315, and a 1/4 wavelength plate 313. The
non-polarization beam splitter 311 performs division without a change in
the polarization state of the light pulse emitted from the transmission
side communication apparatus 20. The polarization beam splitter 312
performs polarization separation on the one of the light pulses split by
the non-polarization beam splitter 311. With respect to the polarization
state of the other light pulse spilt by the non-polarization beam
splitter 311 the 1/4 wavelength plate 313 converts the linear
polarization to the circular polarization and converts the circular
polarization to the linear polarization. The polarization beam splitter
315 performs polarization separation on the light pulse of which the
polarization state is changed by the 1/4 wavelength plate 313.

[0054] The light-receiving unit 32 includes light-receiving devices 321H,
321V, 321R, and 321L. The light-receiving device 321H detects the one
light pulse which is polarization-split by the polarization beam splitter
312, and the light-receiving device 321V detects the other light pulse
which is polarization-split by the polarization beam splitter 312.
Similarly, the light-receiving device 321R detects the one light pulse
which is polarization-split by the polarization beam splitter 315, and
the light-receiving device 321L detects the other light pulse which is
polarization-split by the polarization beam splitter 315.

2-2. Operations of First Embodiment

[Quantum Communication Operation]

[0055] With respect to the quantum communication of the BB84 protocol, the
transmission side communication apparatus 20 performs the following
operations.

[0056] The controller 26 drives the semiconductor light-emitting device
211 of the light source unit 21 by the pulse current to generate the
light pulse. At this time, it is preferable that the number of photons
per pulse be one or less (in the case where the intensity of the light
pulse from the semiconductor light-emitting device 211 is strong, the
number of photons per pulse may be set to be one or less by using a
photo-sensing unit (not shown in this figure) such as an ND filter).

[0057] The light pulse generated by the light source unit 21 is incident
on the liquid crystal retarder 221 of the polarization modulating unit
22. In addition, in the case where the polarizer 225 is installed, the
light pulse is incident through the polarizer 225 on the liquid crystal
retarder 221.

[0058] The liquid crystal retarder 221 is controlled by the controller 26
at random so that the phase difference φ occurring in the
polarization components along the FAST axis and the SLOW axis is one of 0
degrees, 90 degrees, 180 degrees, and 270 degrees according to the
arrival timing of the light pulse.

[0059] With respect to the polarization state of the light pulse passing
through the liquid crystal retarder 221, in the case where the phase
difference φ is 0 degrees, the polarization state is the linear
polarization of the incidence time without a change; in the case where
the phase difference φ is 180 degrees, the polarization state is
changed to the linear polarization perpendicular to the incident linear
polarization; and in the case where the phase difference φ is 90
degrees or 270 degrees, the polarization states are changed to the
circular polarizations which are different from each other in the
direction. In addition, in the case where the phase difference φ is
90 degrees or 270 degrees, whether the polarization states are the
left-handed circular polarization and the right-handed circular
polarization or the right-handed circular polarization and the
left-handed circular polarization is determined according to, the
directions of the optical axes (the SLOW axis and the FAST axis) of the
disposed liquid crystal retarder.

[0060]FIG. 3 illustrates the polarization modulation performed by the
liquid crystal retarder 221. The linear polarization in the x direction
illustrated in FIG. 3 is set to the vertical polarization. In addition,
the FAST axis of the liquid crystal retarder 221 is set to the position
tilted by 45 degrees with respect to the x-direction axis. In addition,
the FAST axis of the liquid crystal retarder 221 is indicated by "F", and
the SLOW axis thereof is indicated by "S".

[0061] In this case, if the phase difference φ occurring in the
polarization components along the FAST axis and the SLOW axis of the
liquid crystal retarder 221 is set to 0 degrees, the light pulse passing
through the liquid crystal retarder 221 becomes the vertical
polarization. In addition, in the case where the phase difference φ
is set to 90 degrees, the light pulse becomes the left-handed circular
polarization; in the case where the phase difference φ is set to 180
degrees, the light pulse becomes the horizontal polarization; and in the
case where the phase difference φ is set to 270 degrees, the light
pulse becomes the right-handed circular polarization.

[0062] In this manner, the light pulse of which the polarization state is
controlled to be one of the four polarization states at random by the
controller 26 is output from the transmission side communication
apparatus 20.

[0063] The reception side communication apparatus 30 allows the
non-polarization beam splitter 311 of the optical unit 31 to divide the
light pulse emitted from the transmission side communication apparatus
20. The one of the light pulses split by the non-polarization beam
splitter 311 is incident on the polarization beam splitter 312, divided
into the polarization components, and incident on the light-receiving
device 321H or the light-receiving device 321V.

[0064] The other of the light pulses split by the non-polarization beam
splitter 311 passes through the 1/4 wavelength plate 313 to allow the
polarization state to be changed, and after that, incident on the
polarization beam splitter 315, divided into the polarization components,
and incident on the light-receiving device 321R or the light-receiving
device 321L. In addition, in the above description, although it is
disclosed that the light pulse is split, in an actual case (herein, it is
assumed that there is no noise), it is not necessary that one light pulse
is detected by all the light-receiving devices. This is because the
intensity of the light pulse is set so that the number of photons per
pulse is one or less, and thus, the photons are detected by one of the
four light-receiving devices to be converted to an electrical signal.

[0065] Table 1 lists the light pulse detection probability of the
light-receiving device of each polarization state. In addition, Table 1
list the values of an ideal case where the number of photons per pulse is
"1", the splitting ratio of the non-polarization beam splitter 311 is
p:(1-p) (herein, 0<p<1), and there is neither optical loss nor
eavesdropping.

[0066] In the case where the light pulse which is the vertical
polarization V or the horizontal polarization H is turned toward the
light-receiving device 321H or the light-receiving device 321V by the
non-polarization beam splitter 311, the probability thereof is "p", and
the light pulses are detected by the corresponding light-receiving
devices. In other words, in the case where the light pulse is the
vertical polarization V, the probability that the light pulse is detected
by the light-receiving device 321V becomes "p", p and the probability
that the light pulse is detected by the light-receiving device 321H
becomes "0". In addition, in the case where the light pulse is the
horizontal polarization H, the probability that the light pulse is
detected by the light-receiving device 321V becomes "0", and the
probability that the light pulse is detected by the light-receiving
device 321H becomes "p".

[0067] In addition, in the case where the light pulse which is the
vertical polarization V or the horizontal polarization H is turned toward
the light-receiving device 321L or the light-receiving device 321R by the
non-polarization beam splitter 311, the probability thereof is "1-p". In
addition, since all the light pulses are detected with the equal
probability of "0.5" by the light-receiving devices, the probability that
the light pulse is detected by the light-receiving devices 321L and 321R
becomes the probability of "0.5(1-p)" in the case where the light pulse
is any one of the vertical polarization V and the horizontal polarization
H.

[0068] Similarly, in the case where the light pulse is the left-handed
circular polarization L, the probability that the light pulse is detected
by the light-receiving device 321L becomes "1-p", and the probability
that the light pulse is detected by the light-receiving device 321R
becomes "0". In addition, in the case where the light pulse is the
right-handed circular polarization R, the probability that the light
pulse is detected by the light-receiving device 321L becomes "0", and the
probability that the light pulse is detected by the light-receiving
device 321R becomes "1-p". In addition, the probability that the light
pulse is detected by the light-receiving devices 321V and 321H becomes
the probability of "0.5p" in the case where the light pulse is any one of
the left-handed circular polarization L and the right-handed circular
polarization R. In the BB84 protocol, the portion which performs the
quantum communication outputs the light reception results of the
light-receiving devices 321V, 321H, 321L, and 321R to the controller 36
by repetitively performing the operations described hereinbefore.

[Classical Communication Operation]

[0069] Next, after the quantum communication of the BB84 protocol, the
classical communication is performed. The transmission side communication
apparatus 20 and the reception side communication apparatus 30 perform
the protocol described hereinafter by using the public communication line
(that is, the communication details are not encrypted but all the
communication details may be recognized by an eavesdropper).

(1) Basis Exchange

[0070] The reception side communication apparatus 30 communicates with the
transmission side communication apparatus 20 through the public
communication line, for example, the classical communication line 55 to
transmit only the information indicating whether the linear polarization
is detected or the circular polarization is detected among the reception
results of the quantum communication from the controller 36 through the
communication unit 35 and the communication unit 25 of the transmission
side communication apparatus 20 to the controller 26. For example, in the
case where the vertical polarization V is detected, the information
indicating "the vertical polarization V is detected" is not transmitted,
but only the information indicating "the linear polarization is detected"
is transmitted. The controller 26 of the transmission side communication
apparatus 20 detects which reception result of which time point is
correct among the reception results and notifies the detection result to
the controller 36 of the reception side communication apparatus 30. The
controller 36 of the reception side communication apparatus 30 selects
only the correct data based on the notified detection result. In other
words, in the case where the transmission side communication apparatus 20
transmits the light pulse which is the linear polarization (the vertical
polarization V or the horizontal polarization H) and the reception side
communication apparatus 30 detects the circular polarization (the
left-handed circular polarization L or the right-handed circular
polarization R), the shared secret information may not be generated. In
addition, in the case where the transmission side communication apparatus
20 transmits the light pulse which is the circular polarization (L or R)
and the reception side communication apparatus 30 detects the linear
polarization (V or H), the shared secret information may not be
generated. Therefore, the data are discarded. In addition, from the
remaining data, for example, in the case of the linear polarization, if
the vertical polarization V is set to "0" and the horizontal polarization
H is set to "1" and in the case of the circular polarization, if the
left-handed circular polarization L is set to "0" and the right-handed
circular polarization R is set to "1", the correlated random bit sequence
may be shared by the transmitting apparatus and the receiving apparatus.
The common key is generated based on the random bit sequence by the
transmission side communication apparatus 20 and the reception side
communication apparatus 30.

[0071] In addition, on the contrary, the transmission side communication
apparatus 20 may transmit only the information indicating "whether the
linear polarization is transmitted or the circular polarization is
transmitted" from the controller 26 through the communication unit 25 and
the communication unit 35 of the transmission side communication
apparatus 30 to the controller 36, and the controller 36 of the reception
side communication apparatus 30 may select only the correct data based on
the notified basis.

[0072] However, in some cases, the bit sequence shared by the transmission
side communication apparatus and the reception side communication
apparatus may include an error caused by the quantum communication line
51 or an error occurring during the transmission and the reception. In
addition, in the case where an eavesdropper who exists on the quantum
communication line 51 taps into the photon information, an error occurs
in the shared bit sequence. Therefore, the estimation of the error rate,
the error correction, or the privacy amplification is performed.

(2) Estimation of Error Rate

[0073] In the estimation of the error rate, in the case where the
transmission side communication apparatus 20 transmits the light pulse
which is the linear polarization (V or H) and the reception side
communication apparatus 30 detects the linear polarization (V or H) and
the case where the transmission side communication apparatus 20 transmits
the light pulse which is the circular polarization (L or R) and the
reception side communication apparatus 30 detects the circular
polarization (L or R), about a half of the data are selected at random
among the bit sequence obtained through the basis exchange. In addition,
the error rate is estimated by combining the values of the data selected
at random. In addition, the data used for the estimation of the error
rate are removed from the bit sequence.

(3) Error Correction

[0074] In the error correction, the error correction is performed on the
bit sequence where the data used for the estimation of the error rate are
removed. For example, in the error correction, the bit sequence is
divided into a plurality of blocks, and a block including an error is
specified by checking the parity of each block, and the error correction
is performed by applying a humming code or the like to the associated
block.

(4) Privacy Amplification

[0075] In the privacy amplification, the privacy amplification is
performed on the error-corrected bit sequence according to the estimated
error rate. At this time, although an eavesdropper does not exist, an
error may occur by the influence of noise in the transmission side
communication apparatus 20, the reception side communication apparatus
30, or the quantum communication line. However, in order to increase the
safety, it is assumed that all errors are originated from eavesdropping.
In other words, under the assumption that an error occurs due to
eavesdropping, the amount of information leaked to the eavesdropper is
estimated from the error rate, and the conversion for reducing the bit
sequence by the amount of information is performed. With respect to the
reduced bit sequence, the amount of information of the eavesdropper may
be neglected.

[0076] By performing this process, for example, if the error rate is small
(for example, equal to or smaller than about 11% in the case of BB84), it
is possible to obtain the bit sequence of which the length is longer than
1. The obtained bit sequence is stored as the common key in the key
memory 23 of the transmission side communication apparatus 20 and the key
memory 33 of the reception side communication apparatus 30. In the case
where the error rate is large and the length of the bit sequence becomes
zero, the key distribution fails.

[0077] In addition, in the above description, for the better
understanding, it is described that the quantum communication portion and
the classical communication portion are sequentially performed. However,
actually, it is preferable that the quantum communication portion be
continuously performed and, when some degrees of the data are stored, the
classical communication portion be intermittently performed at any
necessary time. This is because the amount of the common key which may be
acquired per unit time is increased.

[0078] The common key stored in the transmission side communication
apparatus 20 and the reception side communication apparatus 30 is used at
any time when encryption of communication is necessary. For example, when
the encryption communication is performed by using the common key
encryption system, the amount of plaintext which is encrypted by using
one common key is determined in advance. Herein, if the communication
amount exceeds the determined communication amount, the transmission side
communication apparatus and the reception side communication apparatus
simultaneously extract the common key from the key memories and update
the key used for the common key encryption system. In addition, if the
communication amount is almost constant but it not greatly changed, the
transmission side communication apparatus 20 and the reception side
communication apparatus 30 simultaneously extract the common key from the
key memories at every predetermined time interval and update the key used
for the common key encryption system.

[0079] In this manner, since the transmission side communication apparatus
20 is configured so that the polarization modulating unit is configured
by using the variable wavelength plate, for example, a liquid crystal
retarder, it is possible to easily miniaturize the communication
apparatus and to mount it on a portable electronic apparatus or the like
in comparison with a communication apparatus using a phase modulator or
the like in a quantum encryption communication in the related art.

[0080] In addition, the polarizer is disposed to be integrated with the
liquid crystal retarder in the incidence surface side of the liquid
crystal retarder so that the optical axis of the liquid crystal retarder
is tilted by 45 degrees with respect to the linear polarization emitted
from the polarizer. Therefore, although the position of the polarization
modulating unit is not accurately controlled with respect to the light
source unit, it is possible to set the polarization direction and the
optical axis of the liquid crystal retarder at a desired angle.

3. Second Embodiment

[0081] In a second embodiment, the quantum key distribution is performed
by using a light pulse propagating through a free space according to the
BB84 protocol using the polarization states of the four types of linear
polarizations.

3-1. Configuration of Second Embodiment

[0082]FIG. 4 illustrates a configuration of the second embodiment. In
addition, FIG. 4 illustrates the configuration of the light source unit
21, the polarization modulating unit 22, the optical unit 31, and the
light-receiving unit 32 illustrated in FIG. 1.

[0083] The light source unit 21 of the transmission side communication
apparatus 20 is configured by using a semiconductor light-emitting device
211 such as a laser diode or an LED and a lens 212 which collimates a
light pulse emitted from the semiconductor light-emitting device 211.

[0084] The polarization modulating unit 22 includes a liquid crystal
retarder 221 which converts the polarization state of the collimated
light pulse to one of the four types of polarization states. The liquid
crystal retarder 221 is disposed so that the optical axis is tilted by 45
degrees with respect to the direction of the linear polarization of the
light pulse emitted from the light source unit 21. The liquid crystal
retarder 221 changes the phase difference occurring in the polarization
components along the FAST axis and the SLOW axis according to the control
signal of the controller 26.

[0085] The 1/4 wavelength plate 222 is disposed in the light pulse
emitting surface side of the liquid crystal retarder 221 so as to be
tilted by 45 degrees with respect to the optical axis of the liquid
crystal retarder 221. In the case where the incident light pulse is a
linear polarization, the 1/4 wavelength plate 222 emits the linear
polarization without a change. In addition, in the case where the
incident light pulse is the circular polarization, the 1/4 wavelength
plate 222 emits the changed linear polarization which is tilted by 45
degrees with respect to the linear polarization of the case where the
incident light pulse is the linear polarization.

[0086] In addition, in the polarization modulating unit 22, in the case
where the light emitted from the light source unit 21 is not a linear
polarization, or the case where it is difficult to accurately control the
polarization direction with respect to the optical axis of the liquid
crystal retarder 221 although the light is the linear polarization, a
polarizer 225 is disposed in the light pulse incidence surface side of
the liquid crystal retarder 221. For example, the polarizer 225 is
integrated with the liquid crystal retarder 221 in the light pulse
incidence surface side of the liquid crystal retarder 221 so that the
optical axis of the liquid crystal retarder 221 is set to be tilted by 45
degrees with respect to the linear polarization emitted from the
polarizer 225. According to this configuration of the polarization
modulating unit 22, although the position of the polarization modulating
unit 22 is not accurately controlled with respect to the light source
unit 21, it is possible to set the polarization direction and the optical
axis of the liquid crystal retarder 221 at a desired angle.

[0087] The optical unit 31 of the reception side communication apparatus
30 includes a non-polarization beam splitter 311, and polarization beam
splitters 312 and 315. The non-polarization beam splitter 311 performs
division without a change in the polarization state of the light pulse
emitted from the transmission side communication apparatus 20. The
polarization beam splitter 312 performs polarization separation on the
one of the light pulses split by the non-polarization beam splitter 311.
The polarization beam splitter 315 is installed, for example, so as to be
rotated by 45 degrees around the optical axis of the incident light with
respect to the surface where the polarization beam splitter 312 is
installed, so that the other of the light pulses split by the
non-polarization beam splitter 311 is polarization-separated.

[0088] In addition, as illustrated in FIG. 5, a 1/2 wavelength plate 314
is disposed between the non-polarization beam splitter 311 and the
polarization beam splitter 315, so that the direction of the linear
polarization is rotated by 45 degrees. By doing so, it is not necessary
that the polarization beam splitter 315 is rotated by 45 degrees with
respect to the surface where the polarization beam splitter 312 is
installed.

[0089] The light-receiving unit 32 includes light-receiving device 321H,
321V, 321+, and 321-. The light-receiving device 321H detects the one
light pulse which is polarization-split by the polarization beam splitter
312, and the light-receiving device 321V detects the other light pulse
which is polarization-split by the polarization beam splitter 312.
Similarly, the light-receiving device 321+ detects the one light pulse
which is polarization-split by the polarization beam splitter 315, and
the light-receiving device 321- detects the other light pulse which is
polarization-split by the polarization beam splitter 315.

3-2. Operations of Second Embodiment

[Quantum Communication Operation]

[0090] With respect to the quantum communication of the BB84 protocol, the
transmission side communication apparatus 20 performs the following
operations.

[0091] The controller 26 drives the semiconductor light-emitting device
211 of the light source unit 21 by the pulse current to generate the
light pulse. At this time, it is preferable that the number of photons
per pulse be one or less (in the case where the intensity of the light
pulse from the semiconductor light-emitting device 211 is strong, the
number of photons per pulse may be set to be one or less by using a
photo-sensing unit (not shown in this figure) such as an ND filter).

[0092] The light pulse generated by the light source unit 21 is incident
on the liquid crystal retarder 221 of the polarization modulating unit
22. In addition, in the case where the polarizer 225 is installed, the
light pulse is incident through the polarizer 225 on the liquid crystal
retarder 221.

[0093] The liquid crystal retarder 221 is controlled by the controller 26
at random so that the phase difference occurring in the polarization
components along the FAST axis and the SLOW axis is one of 0 degrees, 90
degrees, 180 degrees, and 270 degrees according to the arrival timing of
the light pulse.

[0094] With respect to the polarization state of the light pulse passing
through the liquid crystal retarder 221, in the case where the phase
difference 0 is 0 degrees, the polarization state is the linear
polarization of the incidence time without a change; in the case where
the phase difference φ is 180 degrees, the polarization state is
changed to the linear polarization perpendicular to the incident linear
polarization; and in the case where the phase difference φ is 90
degrees or 270 degrees, the polarization states are changed to the
circular polarizations which are different from each other in the
direction. In addition, in the case where the phase difference φ is
90 degrees or 270 degrees, whether the polarization states are the
left-handed circular polarization and the right-handed circular
polarization or the right-handed circular polarization and the
left-handed circular polarization is determined according to, the
directions of the optical axes (the SLOW axis and the FAST axis) of the
disposed liquid crystal retarder. The light pulse passing through the
liquid crystal retarder 221 is incident on the 1/4 wavelength plate 222.

[0095] The 1/4 wavelength plate 222 is disposed so as to be tilted by 45
degrees with respect to the optical axis of the liquid crystal retarder
221. Therefore, in the case where the incident light pulse is a linear
polarization, the linear polarization is used without a change; and in
the case where the incident light pulse is a circular polarization, the
light pulse is changed to a linear polarization which is tilted by 45
degrees with respect to the linear polarization.

[0096]FIG. 6 illustrates the polarization modulation performed by the
liquid crystal retarder 221 and the 1/4 wavelength plate 222. The linear
polarization in the x direction illustrated in FIG. 6 is set to the
vertical polarization. In addition, the FAST axis of the liquid crystal
retarder 221 is set to the position tilted by 45 degrees with respect to
the x-direction axis. In addition, the 1/4 wavelength plate 222 is set to
the position tilted by 45 degrees with respect to the optical axis of the
liquid crystal retarder 221. In addition, the FAST axes of the liquid
crystal retarder 221 and the 1/4 wavelength plate 222 are indicated by
"F", and the SLOW axes thereof are indicated by "S".

[0097] In this case, if the phase difference φ occurring in the
polarization components along the FAST axis and the SLOW axis of the
liquid crystal retarder 221 is set to 0 degrees, the light pulse passing
through the liquid crystal retarder 221 becomes the vertical
polarization. In addition, in the case where the phase difference φ
is set to 90 degrees, the light pulse becomes the left-handed circular
polarization; in the case where the phase difference φ is set to 180
degrees, the light pulse becomes the horizontal polarization; and in the
case where the phase difference φ is set to 270 degrees, the light
pulse becomes the right-handed circular polarization.

[0098] Herein, with respect to the angle indicating the polarization
direction of the linear polarization, the perpendicular direction (x-axis
direction) is set to 0 degrees, and the direction of rotation therefrom
in the y-axis direction is set to be positive. Next, after the light
pulse passes through the 1/4 wavelength plate 222, in the case where the
phase difference of the liquid crystal retarder 221 is 0 degrees, the
light pulse becomes the vertical polarization V; in the case where the
phase difference is 90 degrees, the light pulse becomes the
+45-degree-tilted linear polarization +; in the case where the phase
difference is 180 degrees, the light pulse becomes the horizontal
polarization H; and in the case where the phase difference is 270
degrees, the light pulse becomes the -45-degree-tilted linear
polarization -.

[0099] In this manner, the light pulse of which the polarization state is
controlled to be one of the four polarization states at random by the
controller 26 is output from the transmission side communication
apparatus 20.

[0100] The reception side communication apparatus 30 allows the
non-polarization beam splitter 311 of the optical unit 31 to divide the
light pulse emitted from the transmission side communication apparatus
20. The one of the light pulses split by the non-polarization beam
splitter 311 is incident on the polarization beam splitter 312, divided
into the polarization components, and incident on the light-receiving
device 321H or the light-receiving device 321V.

[0101] The other of the light pulses split by the non-polarization beam
splitter 311 is incident on the polarization beam splitter 315, divided
into the polarization components, and incident on the light-receiving
device 321+ or the light-receiving device 321-.

[0102] The light pulse detection probability of the light-receiving device
of each polarization state is listed in Table 2 by replacing and reading
the left-handed circular polarization L, the right-handed circular
polarization R, the light-receiving device 321L, and the light-receiving
device 321R of Table 1 with the linear polarization+, the linear
polarization-, the light-receiving device 321+, and the light-receiving
device 321-, respectively. In addition, Table 2 also lists the values of
an ideal case where the number of photons per pulse is "1", the splitting
ratio of the non-polarization beam splitter 311 is p:(1-p) (herein,
0<p<1), and there is neither optical loss nor eavesdropping.

[0103] In the case where the light pulse which is the vertical
polarization V or the horizontal polarization H is turned toward the
light-receiving device 321H or the light-receiving device 321V by the
non-polarization beam splitter 311, the probability thereof is "p", and
the light pulses are detected by the corresponding light-receiving
devices. In other words, in the case where the light pulse is the
vertical polarization V, the probability that the light pulse is detected
by the light-receiving device 321V becomes "p", and the probability that
the light pulse is detected by the light-receiving device 321H becomes
"0". In addition, in the case where the light pulse is the horizontal
polarization H, the probability that the light pulse is detected by the
light-receiving device 321V becomes "0", and the probability that the
light pulse is detected by the light-receiving device 321H becomes "p".

[0104] In addition, in the case where the light pulse which is the
vertical polarization V or the horizontal polarization H is turned toward
the light-receiving device 321+ or the light-receiving device 321- by the
non-polarization beam splitter 311, the probability thereof is "1-p". In
addition, since all the light pulses are detected with the equal
probability of "0.5" by the light-receiving devices, the probability that
the light pulse is detected by the light-receiving devices 321+ and 321-
becomes the probability of "0.5(1-p)" in the case where the light pulse
is any one of the vertical polarization V and the horizontal polarization
H.

[0105] Similarly, in the case where the light pulse is the +45-degree
linear polarization +, the probability that the light pulse is detected
by the light-receiving device 321+ becomes "1-p", and the probability
that the light pulse is detected by the light-receiving device 321-
becomes "0". In addition, in the case where the light pulse is the
-45-degree linear polarization -, the probability that the light pulse is
detected by the light-receiving device 321+ becomes "0", and the
probability that the light pulse is detected by the light-receiving
device 321- becomes "1-p". In addition, the probability that the light
pulse is detected by the light-receiving devices 321V and 321H becomes
the probability of "0.5p" in the case where the light pulse is any one of
the +45-degree linear polarization + or the -45-degree linear
polarization -. In the BB84 protocol, the portion which performs the
quantum communication repetitively performs the operations described
hereinbefore.

[Classical Communication Operation]

[0106] Next, after the quantum communication of the BB84 protocol, the
classical communication is performed. The transmission side communication
apparatus 20 and the reception side communication apparatus 30 perform
the protocol described hereinafter by using the public communication line
(that is, the communication details are not encrypted but all the
communication details may be recognized by an eavesdropper).

(1) Basis Exchange

[0107] The reception side communication apparatus 30 communicates with the
transmission side communication apparatus 20 through the public
communication line, for example, the classical communication line 55 to
transmit only the information indicating whether the linear polarization
(the vertical polarization V or the horizontal polarization H) is
detected or the 45-degree-tilted linear polarization (the +45-degree
linear polarization + or the -45-degree linear polarization -) is
detected among the reception results of the quantum communication from
the controller 36 through the communication unit 35 and the communication
unit 25 of the transmission side communication apparatus 20 to the
controller 26. For example, in the case where the vertical polarization V
is detected, the information indicating "the vertical polarization V is
detected" is not transmitted, but only the information indicating "the
linear polarization is detected" is transmitted. The controller 26 of the
transmission side communication apparatus 20 detects which reception
result of which time point is correct among the reception results and
notifies the detection result to the controller 36 of the reception side
communication apparatus 30. The controller 36 of the reception side
communication apparatus 30 selects only the correct data based on the
notified detection result. In other words, in the case where the
transmission side communication apparatus 20 transmits the light pulse
which is the linear polarization (V or H) and the reception side
communication apparatus 30 detects the 45-degree-tilted linear
polarization (+ or -), the shared secret information may not be
generated. In addition, in the case where the transmission side
communication apparatus 20 transmits the light pulse which is the
45-degree-tilted linear polarization (+ or -) and the reception side
communication apparatus 30 detects the linear polarization (V or H), the
shared secret information may not be generated. Therefore, the data at
this time are discarded. In addition, from the remaining data, for
example, in the case of the linear polarization, if the vertical
polarization V is set to "0" and the horizontal polarization H is set to
"1" and if the linear polarization +is set to "0" and the linear
polarization - is set to "1", the correlated random bit sequence may be
shared by the transmitting apparatus and the receiving apparatus. The
common key is generated based on the random bit sequence by the
transmission side communication apparatus 20 and the reception side
communication apparatus 30.

[0108] In addition, on the contrary, the transmission side communication
apparatus 20 may transmit only the information indicating "whether the
linear polarization is transmitted or the 45-degree-tilted linear
polarization is transmitted" from the controller 26 through the
communication unit 25 and the communication unit 35 of the transmission
side communication apparatus 30 to the controller 36, and the controller
36 of the reception side communication apparatus 30 may select only the
correct data based on the notified basis.

[0109] In addition, in some cases, the bit sequence shared by the
transmission side communication apparatus and the reception side
communication apparatus may include an error caused by the quantum
communication line 51 or an error occurring during the transmission and
the reception. Therefore, similarly to the first embodiment, the error
correction which corrects errors, the estimation of the error rate, and
the privacy amplification which reduces an amount of the information
which is considered to be subject to eavesdropping based on the estimated
error rate are performed.

[0110] The common key stored in the transmission side communication
apparatus 20 and the reception side communication apparatus 30 is used at
any time when encryption of communication is necessary. For example, when
the encryption communication is performed by using the common key
encryption system, the amount of plaintext which is encrypted by using
one common key is determined in advance. Herein, if the communication
amount exceeds the determined communication amount, the transmission side
communication apparatus and the reception side communication apparatus
simultaneously extract the common key from the key memories and update
the key used for the common key encryption system. In addition, if the
communication amount is almost constant but it not greatly changed, the
transmission side communication apparatus 20 and the reception side
communication apparatus 30 simultaneously extract the common key from the
key memories at every predetermined time interval and update the key used
for the common key encryption system.

[0111] In this manner, in the second embodiment, since the 1/4 wavelength
plate is combined with the liquid crystal retarder, the polarization
modulation which converts the polarization state of the collimated light
pulse to one of the four types of linear polarization states is
performed. In the second embodiment, it is possible to easily miniaturize
the communication apparatus and to mount it on a portable electronic
apparatus or the like in comparison with a communication apparatus using
a phase modulator or the like in the related art.

4. Third Embodiment

[0112] In the quantum encryption communication, for example, as disclosed
in "Secret key can be obtained from both compatible and incompatible
measurements in the six-state QKD protocol" (Matsumoto Ryutaro, IEICE
Technical Report IT2007-43, ISEC20070140, WBS2007-74 (2008-02)) or the
like, the 6-state type of protocol (hereinafter, referred to as a 6-state
protocol) as an extension of the B84 protocol may be used. Therefore, in
the third embodiment, the case of performing the quantum key distribution
by using the six types of polarization states including the four types of
linear polarizations and the two types of circular polarizations is
described.

4-1. Configuration of Third Embodiment

[0113]FIG. 7 illustrates a configuration of a third embodiment. In
addition, FIG. 7 illustrates the configuration of the light source unit
21, the polarization modulating unit 22, the optical unit 31, and the
light-receiving unit 32 illustrated in FIG. 1.

[0114] The light source unit 21 of the transmission side communication
apparatus 20 is configured by using a semiconductor light-emitting device
211 such as a laser diode or an LED and a lens 212 which collimates a
light pulse emitted from the semiconductor light-emitting device 211.

[0115] The polarization modulating unit 22 includes liquid crystal
retarders 221 and 223 which convert the polarization state of the
collimated light pulse to one of the four types of linear polarization
states and the two types of circular polarization states. The liquid
crystal retarder 221 is disposed so that the optical axis is tilted by 45
degrees with respect to the direction of the linear polarization of the
light pulse emitted from the light source unit 21. In addition, the
liquid crystal retarder 223 is disposed so as to be tilted by 45 degrees
with respect to the liquid crystal retarder 223.

[0116] The liquid crystal retarders 221 and 223 change the phase
difference occurring in the polarization components along the FAST axis
and the SLOW axis according to the control signal of the controller 26.

[0117] In addition, in the polarization modulating unit 22, in the case
where the light emitted from the light source unit 21 is not a linear
polarization, or the case where it is difficult to accurately control the
polarization direction with respect to the optical axis of the liquid
crystal retarder 221 although the light is the linear polarization, a
polarizer 225 is disposed in the light pulse incidence surface side of
the liquid crystal retarder 221. For example, the polarizer 225 is
integrated with the liquid crystal retarder 221 in the light pulse
incidence surface side of the liquid crystal retarder 221 so that the
optical axis of the liquid crystal retarder 221 is set to be tilted by 45
degrees with respect to the linear polarization emitted from the
polarizer 225. According to this configuration of the polarization
modulating unit 22, although the position of the polarization modulating
unit 22 is not accurately controlled with respect to the light source
unit 21, it is possible to set the polarization direction and the optical
axis of the liquid crystal retarder 221 at a desired angle.

[0118] The optical unit 31 of the reception side communication apparatus
30 includes non-polarization beam splitters 310 and 311, polarization
beam splitters 312, 315, and 316, and 1/4 wavelength plate 313. The
non-polarization beam splitter 310 performs division without a change in
the polarization state of the light pulse emitted from the transmission
side communication apparatus 20. The non-polarization beam splitter 311
divides the one of the light pulses split by the non-polarization beam
splitter 310 without a change in the polarization state. The polarization
beam splitter 312 performs polarization separation on the one of the
light pulses split by the non-polarization beam splitter 311 into
polarizations. With respect to the polarization state of the other light
pulse spilt by the non-polarization beam splitter 311, the 1/4 wavelength
plate 313 converts the linear polarization to the circular polarization
and converts the circular polarization to the linear polarization. The
polarization beam splitter 315 performs the polarization separation on
the light pulse of which the polarization state is changed by the 1/4
wavelength plate 313.

[0119] The polarization beam splitter 316 is installed, for example, so as
to be rotated around the incident light as an axis to be tilted by 45
degrees with respect to the surface where the polarization beam splitter
312 is installed, so that the light pulse is split into two linear
polarizations in the directions which are tilted by 45 degrees with
respect to the polarization beam splitter 312. In addition, a 1/2
wavelength plate 317 is disposed between the non-polarization beam
splitter 310 and the polarization beam splitter 316, so that the
direction of the linear polarization is rotated by 45 degrees. Therefore,
it is not necessary that the polarization beam splitter 316 is installed
to be tilted by 45 degrees. In addition, which one of the three light
pulses split by the non-polarization beam splitters 310 and 311 is
polarization-separated by which one of the polarization beam splitters
may not be the same as that illustrated.

[0120] The light-receiving unit 32 includes light-receiving devices 321H,
321V, 321L, 321R, 321+, and 321-. The light-receiving device 321H detects
the one light pulse which is polarization-split by the polarization beam
splitter 312, and the light-receiving device 321V detects the other light
pulse which is polarization-split by the polarization beam splitter 312.
Similarly, the light-receiving device 321L detects the one light pulse
which is polarization-split by the polarization beam splitter 315, and
the light-receiving device 321R detects the other light pulse which is
polarization-split by the polarization beam splitter 315. In addition,
the light-receiving device 321+ detects the one light pulse which is
polarization-split by the polarization beam splitter 316, and the
light-receiving device 321- detects the other light pulse which is
polarization-split by the polarization beam splitter 316.

4-2. Operations of Third Embodiment

[Quantum Communication Operation]

[0121] With respect to the quantum communication of the E-state protocol,
the transmission side communication apparatus 20 performs the following
operations.

[0122] The controller 26 drives the semiconductor light-emitting device
211 of the light source unit 21 by the pulse current to generate the
light pulse. At this time, it is preferable that the number of photons
per pulse be one or less (in the case where the intensity of the light
pulse from the semiconductor light-emitting device 211 is strong, the
number of photons per pulse may be set to be one or less by using a
photo-sensing unit (not shown in this figure) such as an ND filter).

[0123] The light pulse generated by the light source unit 21 is incident
on the liquid crystal retarder 221 of the polarization modulating unit
22. In addition, in the case where the polarizer 225 is installed, the
light pulse is incident through the polarizer 225 on the liquid crystal
retarder 221.

[0124] The liquid crystal retarder 221 is controlled by the controller 26
at random so that the phase difference occurring in the polarization
components along the FAST axis and the SLOW axis is one of 0 degrees, 90
degrees, 180 degrees, and 270 degrees according to the arrival timing of
the light pulse.

[0125] With respect to the polarization state of the light pulse passing
through the liquid crystal retarder 221, in the case where the phase
difference φ is 0 degrees, the polarization state is the linear
polarization of the incidence time without a change; in the case where
the phase difference φ is 180 degrees, the polarization state is
changed to the linear polarization perpendicular to the incident linear
polarization; and in the case where the phase difference φ is 90
degrees or 270 degrees, the polarization states are changed to the
circular polarizations which are different from each other in the
direction. In addition, in the case where the phase difference φ is
90 degrees or 270 degrees, whether the polarization states are the
left-handed circular polarization and the right-handed circular
polarization or the right-handed circular polarization and the
left-handed circular polarization is determined according to, the
directions of the optical axes (the SLOW axis and the FAST axis) of the
disposed liquid crystal retarder. The light pulse passing through the
liquid crystal retarder 221 is incident on the liquid crystal retarder
223.

[0126] The liquid crystal retarder 223 is disposed so as to be tilted by
45 degrees with respect to the optical axis of the liquid crystal
retarder 221, and the liquid crystal retarder 223 is controlled by the
controller 26 at random so that the phase difference θ occurring in
the polarization components along the FAST axis and the SLOW axis of the
liquid crystal retarder 223 is one of 0 degrees and 90 degrees.

[0127] In the case where the phase difference 0 of the liquid crystal
retarder 223 is 0 degrees, the polarization state of the light pulse
passing through the liquid crystal retarder 223 is not changed. In
addition, in the case where the phase difference θ of the liquid
crystal retarder 223 is 90 degrees, the incident light pulse which is a
linear polarization is the linear polarization without a change, and the
incident light pulse which is a circular polarization is changed to a
linear polarization which is tilted by 45 degrees with respect to the
linear polarization.

[0128]FIG. 8 illustrates the polarization modulation performed by the
liquid crystal retarders 221 and 223. The linear polarization in the x
direction illustrated in FIG. 8 is set to the vertical polarization. In
addition, the FAST axis of the liquid crystal retarder 221 is set to the
position tilted by 45 degrees with respect to the x-direction axis. In
addition, the liquid crystal retarder 223 is set to the position tilted
by 45 degrees with respect to the optical axis of the liquid crystal
retarder 221. In addition, the FAST axes of he liquid crystal retarders
221 and 223 are indicated by "F", and the SLOW axes thereof are indicated
by "S".

[0129] In this case, if the phase difference φ occurring in the
polarization components along the FAST axis and the SLOW axis of the
liquid crystal retarder 221 is set to 0 degrees, the light pulse passing
through the liquid crystal retarder 221 becomes the vertical
polarization. In addition, in the case where the phase difference φ
is set to 90 degrees, the light pulse becomes the left-handed circular
polarization; in the case where the phase difference φ is set to 180
degrees, the light pulse becomes the horizontal polarization; and in the
case where the phase difference φ is set to 270 degrees, the light
pulse becomes the right-handed circular polarization.

[0130] Herein, with respect to the angle indicating the polarization
direction of the linear polarization, the perpendicular direction (x-axis
direction) is set to 0 degrees, and the direction of rotation therefrom
in the y-axis direction is set to be positive. After the light pulse
passes through the liquid crystal retarder 223, in the case where the
phase difference θ of the liquid crystal retarder 2 is 0 degrees,
the polarization state is not changed. Therefore, in the case where the
phase difference of the liquid crystal retarder 221 is 0 degrees, the
light pulse becomes the vertical polarization; in the case where the
phase difference φ is 90 degrees, the light pulse becomes the
left-handed circular polarization; in the case where the phase difference
φ is 180 degrees, the light pulse becomes the horizontal
polarization, in the case where the phase difference φ is 270
degrees, the light pulse becomes the right-handed circular polarization.

[0131] In addition, when the phase difference θ of the liquid
crystal retarder 223 is 90 degrees, in the case where the phase
difference φ of the liquid crystal retarder 221 is 0 degrees, the
light pulse becomes the vertical polarization; in the case where the
phase difference φ is 90 degrees, the light pulse becomes the
+45-degree-tilted linear polarization; in the case where the phase
difference φ is 180 degrees, the light pulse becomes the horizontal
polarization; and in the case where the phase difference φ is 270
degrees, the light pulse becomes the -45-degree-tilted linear
polarization. In addition, Table 3 illustrates a relationship among the
phase difference φ of the liquid crystal retarder 221, the phase
difference 8 of the liquid crystal retarder 223, and the
polarization-modulated polarization state.

[0132] In this manner, the light pulse of which the polarization state is
controlled to be one of the six polarization states at random by the
controller 26 is output from the transmission side communication
apparatus 20.

[0133] The reception side communication apparatus 30 allows the
non-polarization beam splitter 310 of the optical unit 31 to divide the
light pulse emitted from the transmission side communication apparatus
20. The one of the light pulses split by the non-polarization beam
splitter 310 is incident on the non-polarization beam splitter 311 to be
further divided. The one of the light pulses split by the
non-polarization beam splitter 311 is incident on the polarization beam
splitter 312, divided into the polarization components, and incident on
the light-receiving device 321H or the light-receiving device 321V.

[0134] The other of the light pulses split by the non-polarization beam
splitter 311 passes through the 1/4 wavelength plate 313 to allow the
polarization state to be changed, and after that, divided by the
polarization beam splitter 315. The split light pulse is incident on the
polarization beam splitter 315, divided into the polarization components,
and incident on the light-receiving device 321L or the light-receiving
device 321R.

[0135] The other of the light pulses split by the non-polarization beam
splitter 310 is incident on the polarization beam splitter 316 which
divides the light pulse into the two linear polarizations in the
45-degree-tilted directions, divided into the polarization components,
and incident on the light-receiving device 321+ or the light-receiving
device 321-.

[0136] For the convenience of the description, in the above disclosure,
although it is described that "the light pulse is split", in an actual
case, it is not necessary that one light pulse is detected by all the
light-receiving devices. This is because the intensity of the light pulse
is set so that the number of photons per pulse is one or less, and thus,
the photons are detected by one of the six light-receiving devices to be
converted to an electrical signal.

[0137] Similarly to the first and second embodiments, the light pulse
detection probability of the light-receiving device of each polarization
state is listed in Table. In addition, in Table 4, the number of photons
per pulse is "1"; the splitting ratio of the non-polarization beam
splitter 310 is p:(1-p) (herein, 0<p<1); and the splitting ratio of
the non-polarization beam splitter 311 is q:(1-q) (herein, 0<q<1).
In addition, Table 4 lists the values of an ideal case where there is
neither optical loss nor eavesdropping.

[0138] In the case where the light pulse which is the vertical
polarization V or the horizontal polarization H is turned toward the
light-receiving device 321H or the light-receiving device 321V by the
non-polarization beam splitter 311, the probability thereof is "pq", and
the light pulses are detected by the corresponding light-receiving
devices. In other words, in the case where the light pulse is the
vertical polarization V, the probability that the light pulse is detected
by the light-receiving device 321V becomes. "pq", and the probability
that the light pulse is detected by the light-receiving device 321H
becomes "0". In addition, in the case where the light pulse is the
horizontal polarization H, the probability that the light pulse is
detected by the light-receiving device 321V becomes "0", and the
probability that the light pulse is detected by the light-receiving
device 321H becomes "pq".

[0139] In addition, in the case where the light pulse which is the
vertical polarization V or the horizontal polarization H is turned toward
the light-receiving device 321L or the light-receiving device 321R by the
non-polarization beam splitter 311, the probability thereof is "p(1-q)".
In addition, since all the light pulses are detected with the equal
probability of "0.5" by the light-receiving devices, the probability that
the light pulse is detected by the light-receiving devices 321L and 321R
becomes the probability of "0.5p(1-q)" in the case where the light pulse
is any one of the vertical polarization V and the horizontal polarization
H.

[0140] In addition, in the case where the light pulse which is the
vertical polarization V or the horizontal polarization H is turned toward
the light-receiving device 321+ or the light-receiving device 321- by the
non-polarization beam splitter 310, the probability thereof is "1-p". In
addition, since all the light pulses are detected with the equal
probability of "0.5" by the light-receiving devices, the probability that
the light pulse is detected by the light-receiving devices 321+ and 321-
becomes the probability of "0.5(1-p)" in the case where the light pulse
is any one of the vertical polarization V and the horizontal polarization
H.

[0141] Similarly, in the case where the light pulse is the left-handed
circular polarization L, the probability that the light pulse is detected
by the light-receiving device 321L becomes "p(1-q)", and the probability
that the light pulse is detected by the light-receiving device 321R
becomes "0". In addition, in the case where the light pulse is the
right-handed circular polarization R, the probability that the light
pulse is detected by the light-receiving device 321L becomes "0", and the
probability that the light pulse is detected by the light-receiving
device 321R becomes "p(1-q)". In addition, the probability that the light
pulse is detected by the light-receiving devices 321V and 321H becomes
the probability of "0.5pq" in the case where the light pulse is any one
of the left-handed circular polarization L and the right-handed circular
polarization R. Therefore, the probability that the light pulse is
detected by the light-receiving devices 321+ and 321- becomes the
probability of "0.5(1-p)" in the case where the light pulse is any one of
the left-handed circular polarization L and the right-handed circular
polarization R.

[0142] In addition, in the case where the light pulse is the linear
polarization +, the probability that the light pulse is detected by the
light-receiving device 321+ becomes "1-p", and the probability that the
light pulse is detected by the light-receiving device 321- becomes "0".
In addition, in the case where the light pulse is the linear polarization
- the probability that the light pulse is detected by the light-receiving
device 321+ becomes "0", and the probability that the light pulse is
detected by the light-receiving device 321- becomes "1-p". In addition,
the probability that the light pulse is detected by the light-receiving
devices 321V and 321H becomes the probability of "0.5pq" in the case
where the light pulse is any one of the linear polarization + and the
linear polarization -. Therefore, the probability that the light pulse is
detected by the light-receiving devices 321L and 321R becomes the
probability of "0.5p(1-q)" in the case where the light pulse is any one
of the linear polarization + and the linear polarization -. In the
6-state protocol, the portion which performs the quantum communication
repetitively performs the operations described hereinbefore.

[Classical Communication Operation]

[0143] Next, after the quantum communication of the 6-state protocol, the
classical communication is performed. The transmission side communication
apparatus 20 and the reception side communication apparatus 30 perform
the protocol described hereinafter by using the public communication line
(that is, the communication details are not encrypted but all the
communication details may be recognized by an eavesdropper).

(1) Basis Exchange

[0144] The reception side communication apparatus 30 communicates with the
transmission side communication apparatus 20 through the public
communication line, for example, the classical communication line 55 to
transmit only the information indicating whether any one of the linear
polarization (the vertical polarization V or the horizontal polarization
H), the 45-degree-tilted linear polarization (the +45-degree linear
polarization+ or the -45-degree linear polarization-), and the circular
polarization (the left-handed circular polarization L or the right-handed
circular polarization R) is detected among the reception results of the
quantum communication from the controller 36 through the communication
unit 35 and the communication unit 25 of the transmission side
communication apparatus 20 to the controller 26. For example, in the case
where the vertical polarization V is detected, the information indicating
"the vertical polarization V is detected" is not transmitted, but only
the information indicating "the linear polarization is detected" is
transmitted. The controller 26 of the transmission side communication
apparatus 20 detects which reception result of which time point is
correct among the reception results and notifies the detection result to
the controller 36 of the reception side communication apparatus 30. The
controller 36 of the reception side communication apparatus 30 selects
only the correct data based on the notified detection result. In other
words, in the case where the transmission side communication apparatus 20
transmits the light pulse which is the linear polarization (V or H) and
the reception side communication apparatus 30 detects the circular
polarization (L or R) or the 45-degree-tilted linear polarization (+ or
-), since the shared secret information may not be generated, the data
are discarded. In addition, in the case where the transmission side
communication apparatus 20 transmits the light pulse which is the
circular polarization (L or R) and the reception side communication
apparatus 30 detects the linear polarization (V or H) or the
45-degree-tilted linear polarization (+ or -), since the shared secret
information may not generated, the data are discarded. In addition, in
the case where the transmission side communication apparatus 20 transmits
the light pulse which is the 45-degree-tilted linear polarization (+ or
-) and the reception side communication apparatus 30 detects the linear
polarization (V or H) or the circular polarization (L or R), since the
shared secret information may not be generated, the data are discarded.

[0145] In addition, from the remaining data, for example, in the case of
the linear polarization, if the vertical polarization V is set to "0" and
the horizontal polarization H is set to "1"; in the case of the circular
polarization, if the left-handed circular polarization L is set to "0"
and the right-handed circular polarization R is set to "1"; and in the
case of the 45-degree-tilted linear polarization, if the +45-degree
linear polarization + is set to "0" and the -45-degree linear
polarization - is set to "1". By doing so, the correlated random bit
sequence may be shared by the transmission side communication apparatus
and the reception side communication apparatus. The common key is
generated based on the random bit sequence by the transmission side
communication apparatus 20 and the reception side communication apparatus
30.

[0146] In addition, on the contrary, the transmission side communication
apparatus 20 may transmit only the information indicating "which one of
the linear polarization, the 45-degree-tilted linear polarization, and
the circular polarization is transmitted from the controller 26 through
the communication unit 25 and the communication unit 35 of the
transmission side communication apparatus 30 to the controller 36, and
the controller 36 of the reception side communication apparatus 30 may
select only the correct data based on the notified basis.

(2) Estimation of Error Rate

[0147] In the case where the transmission side communication apparatus 20
transmits the light pulse which is the linear polarization (V or H) and
the reception side communication apparatus 30 detects the linear
polarization (V or H), the case where the transmission side communication
apparatus 20 transmits the light pulse which is the circular polarization
(L or R) and the reception side communication apparatus 30 detects the
circular polarization (L or R), and the case where the transmission side
communication apparatus 20 transmits the light pulse which is the
45-degree-tilted linear polarization (+ or -) and the reception side
communication apparatus 30 detects the 45-degree-tilted linear
polarization (+ or -), about a half of the data are selected at random
among the bit sequence obtained through the basis exchange, and the error
rate is estimated by combining the values there by the transmission side
communication apparatus 20 and the reception side communication apparatus
30. The data used at this time are removed.

(3) Error Correction

[0148] In the error correction, similarly to the first embodiment or the
like, the error correction is performed on the bit sequence where the
data used for the estimation of the error rate are removed.

(4) Privacy Amplification

[0149] In the privacy amplification, similarly to the first embodiment or
the like, the privacy amplification is performed on the error-corrected
bit sequence according to the estimated error rate. Herein, in the case
where the error rate is large and the length of the bit sequence becomes
zero, the key distribution fails. In addition, if the error rate is small
(for example, equal to or smaller than about 12.6% in the case of the
6-state protocol), it is possible to obtain the bit sequence of which the
length is longer than 1. The obtained bit sequence is stored as the
common key in the key memory 23 of the transmission side communication
apparatus 20 and the key memory 33 of the reception side communication
apparatus 30.

[0150] In this manner, in the third embodiment, the liquid crystal
retarder 223 of which the optical axis is tilted by 45 degrees with
respect to the optical axis of the liquid crystal retarder 221 is
disposed in the emitting surface side of the liquid crystal retarder 221.
Therefore, the phase difference is changed by the liquid crystal
retarders 221 and 223, so that the polarization state of the collimated
light pulse may be converted to one of the six types of polarization
states. Therefore, even in the case where the 6-state type protocol is
used, it is possible to easily miniaturize the communication apparatus
and to mount it on a portable electronic apparatus or the like.

5. Fourth Embodiment

[0151] In the quantum encryption communication, for example, as disclosed
in Japanese Unexamined Patent Application Publication No. 2007-286551,
the decoy method capable of further increasing the encrypted intensity of
the quantum key distribution by transmitting the light pulse which the
intensity modulation is performed on is used. Therefore, in a fourth
embodiment, a case of performing the quantum key distribution by using
the decoy method in the BB84 protocol using the four types of
polarization states including the two types of linear polarizations and
the two types of circular polarizations is described.

5-1. Configuration of Fourth Embodiment

[0152]FIG. 9 illustrates a configuration of the fourth embodiment. In
addition, FIG. 9 illustrates the configuration of the light source unit
21, the polarization modulating unit 22, the optical unit 31, and the
light-receiving unit 32 illustrated in FIG. 1.

[0153] The light source unit 21 of the transmission side communication
apparatus 20 is configured by using a semiconductor light-emitting device
211 such as a laser diode or an LED and a lens 212 which collimates a
light pulse emitted from the semiconductor light-emitting device 211.

[0154] The polarization modulating unit 22 includes a liquid crystal
retarder 221 which converts the polarization state of the collimated
light pulse to one of the four types of polarization states. The liquid
crystal retarder 221 is disposed so that the optical axis is tilted by 45
degrees with respect to the polarization axis of the polarizer 224b of
the later-described intensity modulator 224. The liquid crystal retarder
221 changes the phase difference occurring in the polarization components
along the FAST axis and the SLOW axis according to the control signal of
the controller 26.

[0155] An intensity modulator 224 which performs the intensity modulation
of the light pulse is disposed in the incidence surface side of the
liquid crystal retarder 221. The intensity modulator 224 is configured by
using a liquid crystal retarder 224a and a polarizer 224b. The liquid
crystal retarder 224a is disposed so that the optical axis of the liquid
crystal retarder 224a is tilted by 45 degrees with respect to the
direction of the linear polarization of the incident light pulse. In the
case where the light emitted from the light source unit 21 is the linear
polarization, the polarization axis of the polarizer 224b is set to the
direction perpendicular to the polarization direction.

[0156] The liquid crystal retarder 224a changes the phase difference
occurring in the polarization components along the FAST axis and the SLOW
axis according to the control signal of the controller 26. In this
manner, the phase difference is changed, so-that the intensity of the
light pulse output through the polarizer 224b is adjusted.

[0157] In addition, in the fourth embodiment, although the intensity
modulator 224 is implemented with an optical system including the liquid
crystal retarder 224a and the polarizer 224b, the intensity modulation
may be performed by modulating a current value for driving the
semiconductor light-emitting device 211. In addition, an optical
intensity modulator using a non-linear optical crystal or the like may
also be used.

[0158] In addition, in the polarization modulating unit 22, in the case
where the light emitted from the light source unit 21 is not the linear
polarization or the case where it is difficult to accurately control the
polarization direction with respect to the optical axis of the liquid
crystal retarder 224a although the light is the linear polarization, a
polarizer 225 is disposed in the light pulse incidence surface side of
the liquid crystal retarder 224a. For example, the polarizer 225 is
disposed in the light pulse incidence surface side of the intensity
modulator 224, the position of the polarizer 225 is set so that the
optical axis of the liquid crystal retarder 224a is tilted by 45 degrees
with respect to the linear polarization, and the intensity modulator 224
and the polarizer 225 are integrated with the liquid crystal retarder
221. According to this configuration of the polarization modulating unit
22, although the position of the polarization modulating unit 22 is not
accurately controlled with respect to the light source unit 21, it is
possible to set the polarization direction and the optical axis of the
liquid crystal retarder 224a or the like at a desired angle.

[0159] The optical unit 31 of the reception side communication apparatus
30 includes a non-polarization beam splitter 311, polarization beam
splitters 312 and 315, and a 1/4 wavelength plate 313. The
non-polarization beam splitter 311 performs division without a change in
the polarization state of the light pulse emitted from the transmission
side communication apparatus 20. The polarization beam splitter 312
performs the polarization separation on the one of the light pulses split
by the non-polarization beam splitter 311. With respect to the
polarization state of the other light pulse spilt by the non-polarization
beam splitter 311, the 1/4 wavelength plate 313 converts the linear
polarization to the circular polarization and converts the circular
polarization to the linear polarization. The polarization beam splitter
315 performs the polarization separation on the light pulse of which the
polarization state is changed by the 1/4 wavelength plate 313.

[0160] The light-receiving unit 32 includes light-receiving devices 321H,
321V, 321R, and 321L. The light-receiving device 321H detects the one
light pulse which is polarization-split by the polarization beam splitter
312, and the light-receiving device 321V detects the other light pulse
which is polarization-split by the polarization beam splitter 312.
Similarly, the light-receiving device 321R detects the one light pulse
which is polarization-split by the polarization beam splitter 315, and
the light-receiving device 321L detects the other light pulse which is
polarization-split by the polarization beam splitter 315.

5-2. Operations of Fourth Embodiment

[Quantum Communication Operation]

[0161] In the quantum communication of the BB84 protocol combined with the
decoy method, the transmission side communication apparatus 20 performs
the following operations.

[0162] The controller 26 drives the semiconductor light-emitting device
211 of the light source unit 21 by the pulse current to generate the
light pulse. The light pulse generated by the light source unit 21 is
incident on the intensity modulator 224 of the polarization modulating
unit 22. In addition, in the case where the polarizer 225 is installed,
the light pulse is incident through the polarizer 225 on intensity
modulator 224.

[0163] The liquid crystal retarder 224a of the intensity modulator 224 is
controlled at random so that the phase difference a occurring in the
polarization components along the FAST axis and the SLOW axis is one
value among a plurality of predetermined values according to the arrival
timing of the light pulse.

[0164] The light pulse passing through the liquid crystal retarder 224a is
incident on the polarizer 224b. In the case where the phase difference a
is 0 degrees, since the liquid crystal retarder 224a does not influence
the polarization of the incident light pulse, the light pulse with the
polarization state of the light pulse emitted from the liquid crystal
retarder 224a, which is the linear polarization of the incidence time
without a change, is incident on the polarizer 224b.

[0165] In the case where the light emitted from the light source unit 21
is the linear polarization, the polarization axis of the polarizer 224b
is set to be directed to the direction perpendicular to the polarization
direction; and in the case where the polarizer 225 is installed, the
polarization axis of the polarizer 224b is set to be directed to the
polarization axis of the polarizer 225. Therefore, in the phase
difference α in the liquid crystal retarder 224a is 0 degrees, the
light pulse may not pass through the polarizer 224b, and the intensity of
the light pulse becomes zero.

[0166] In the case where the phase difference α is not 0 degrees,
since the liquid crystal retarder 224a influences the polarization of the
incident light pulse, the light pulse with the polarization state of the
light pulse emitted from the liquid crystal retarder 224a, which is
generally changed to, the elliptical polarization, is incident on the
polarizer 224b.

[0167] Although the light pulse incident on the polarizer 224b is emitted
as the linear polarization, the intensity of the light pulse emitted at
this time is changed depending on the polarization state of the
elliptical polarization at the incidence time, that is, the phase
difference α. For example, if the phase difference α is 90
degrees, the light pulse passing through the liquid crystal retarder 3
becomes the circular polarization; and if the light pulse passes through
the polarizer 224b, the intensity of the light pulse becomes 1/2 of the
original intensity.

[0168] For example, if the phase difference α is 180 degrees, the
light pulse passing through the liquid crystal retarder 3 becomes the
linear polarization which is polarized in the direction perpendicular to
the incident linear polarization, and since the light pulse passes
through the polarizer 224b as it is, the intensity of the light pulse is
not changed.

[0169] In other words, if the phase difference occurring in the
polarization components along the FAST axis and the SLOW axis of the
liquid crystal retarder 224a is set to α, the intensity modulator
224 which is configured with an optical system formed by combining the
liquid crystal retarder 224a and the polarizer 224b changes the intensity
of the incident light pulse to a product of [sin(α/2)] 2
thereof.

[0170] With respect to a plurality of predetermined values of the phase
difference α, the average value of the number of photons including
one light pulse after the passing through the intensity modulator 224 may
be defined, for example, to be zero, one, or ten. The light pulse passing
through the intensity modulator 224 is incident on the liquid crystal
retarder 221. The liquid crystal retarder 221 performs the same
operations as those of the first embodiment, and the light pulse of which
the polarization state is controlled to be one of the four polarization
states at random by the controller 26 is output from the transmission
side communication apparatus 20.

[0171] The reception side communication apparatus 30 performs the same
operations as those of the first embodiment.

[Classical Communication Operation]

[0172] Next, after the quantum communication of the BB84 protocol combined
with the decoy method, the classical communication is performed.
Similarly to the first embodiment, the transmission side communication
apparatus 20 and the reception side communication apparatus 30 perform
(1) the basis exchange, (3) the error correction, and (4) the privacy
amplification by using the public communication line. In addition, (2)
the estimation of the error rate is performed as follows.

(2) Estimation of Error Rate

[0173] In the case where the transmission side communication apparatus 20
transmits the light pulse which is the linear polarization (V or H) and
the reception side communication apparatus 30 detects the linear
polarization (V or H) or the case where the transmission side
communication apparatus 20 transmits the light pulse which is the
circular polarization (L or R) and the reception side communication
apparatus 30 detects the circular polarization (L or R), about a half of
the data are selected at random among the bit sequence obtained through
the basis exchange. In addition, the error rate is estimated by combining
the values by the transmission side communication apparatus 20 and the
reception side communication apparatus 30.

[0174] At this time, with respect to each of the light pulses which are
modulated with different intensities, the error rate is estimated by the
intensity modulator 224 of the transmission side communication apparatus
20. For example, in the case where the average value of the number of
photons included in one light pulse is set to be zero, one, and ten, the
error rate of the pulse of which the average value of the number of
photons is zero, the error rate of the pulse of which the average value
of the number of photons is one, and the error rate of the pulse of which
the average value of the number of photons is ten are estimated. The data
used at this time are discarded. The error rate is a parameter which is
necessary for (4) privacy amplification.

[0175] Since the error rate is estimated in the relation to each of the
light pulses which have different average numbers of photons included, in
the case where all errors are assumed to be caused by eavesdropping, it
is difficult to estimate the mutual information amount leaked to the
eavesdropper in comparison with the first embodiment. Therefore, if other
conditions are the same, it is possible to generate much more common keys
in comparison with the first embodiment.

[0176] In addition, in the fourth embodiment, the decoy method may be
combined with the BB84 protocol using the polarization states of the four
types of linear polarizations or the 6-state protocol using the
polarization states of the four types of linear polarizations and the two
types of circular polarizations.

[0177] In this manner, in the fourth embodiment, an intensity modulator
224 configured with, a liquid crystal retarder 224a and a polarizer 224b
is disposed in the incidence surface side of the liquid crystal retarder
221. Therefore, the conversion of the intensity of the light pulse is
performed by controlling the liquid crystal retarder 224a, and the
conversion of the polarization state is performed by controlling the
liquid crystal retarder 221, so that it is possible to further increase
the encrypted intensity of the quantum key distribution. In other words,
even in the case of using the decoy method, it is possible to easily
miniaturize the communication apparatus and to mount it on a portable
electronic apparatus or the like.

6. Fifth Embodiment

[0178] In the aforementioned first to fourth embodiments, the cases of
performing the quantum encryption communication by using a free space as
the quantum communication line 51 are described. In this manner, in the
case where the free space is used as the quantum communication line 51,
the position alignment is necessary so that the light pulse emitted from
the transmission side communication apparatus 20 is correctly received by
the reception side communication apparatus 30. However, if the optical
fiber is used as the quantum communication line, the position alignment
of the transmission side communication apparatus 20 and the reception
side communication apparatus 30 is not necessary, so that it is possible
to easily perform the quantum encryption communication. In a fifth
embodiment, a case of using an optical fiber 52 as the quantum
communication line is described.

[0179] In addition, in the fifth embodiment, a case of performing the
quantum key distribution by using the decoy method in the BB84 protocol
using the four types of polarization states including the two types of
linear polarizations and the two types of circular polarizations is
described.

6-1. Configuration of Fifth Embodiment

[0180]FIG. 10 illustrates a configuration of the fifth embodiment. In
addition, FIG. 10 illustrates the configuration of the light source unit
21, the polarization modulating unit 22, the optical unit 31, the
light-receiving unit 32, and the like illustrated in FIG. 1.

[0181] The light source unit 21 of the transmission side communication
apparatus 20 is configured by using a semiconductor light-emitting device
211 such as a laser diode or an LED and a lens 212 which collimates a
light pulse emitted from the semiconductor light-emitting device 211.

[0182] The polarization modulating unit 22 includes a liquid crystal
retarder 221 which converts the polarization state of the collimated
light pulse to one of the four types of polarization states. The liquid
crystal retarder 221 is disposed so that the optical axis is tilted by 45
degrees with respect to the polarization axis of the polarizer 224b of
the later-described intensity modulator 224. The liquid crystal retarder
221 is controlled by the controller 26 to change the phase difference
occurring in the polarization components along the FAST axis and the SLOW
axis according to the voltage applied to the controller 26.

[0183] An intensity modulator 224 which performs the intensity modulation
of the light pulse is disposed in the incidence surface side of the
liquid crystal retarder 221. The intensity modulator 224 is configured by
using a liquid crystal retarder 224a and a polarizer 224b. The liquid
crystal retarder 224a is disposed so that the optical axis of the liquid
crystal retarder 224a is tilted by 45 degrees with respect to the
direction of the linear polarization of the incident light pulse. In the
case where the light emitted from the light source unit 21 is the linear
polarization, the polarization axis of the polarizer 224b is set to the
direction perpendicular to the polarization direction.

[0184] The liquid crystal retarder 224a changes the phase difference
occurring in the polarization components along the FAST axis and the SLOW
axis according to the control signal of the controller 26. In this
manner, the phase difference is changed, so that the intensity of the
light pulse output through the polarizer 224b is adjusted.

[0185] The light pulse emitted from the liquid crystal retarder 221 is
condensed by the lens 226 and incident on the optical fiber 52 connected
to the optical fiber connector 27.

[0186] In addition, in the fifth embodiment, although the intensity
modulator 224 is implemented with an optical system including the liquid
crystal retarder 224a and the polarizer 224b, the intensity modulation
may be performed by modulating a current value for driving the
semiconductor light-emitting device 211. In addition, an optical
intensity modulator using a non-linear optical crystal or the like may
also be used.

[0187] In addition, in the polarization modulating unit 22, in the case
where the light emitted from the light source unit 21 is not the linear
polarization or the case where it is difficult to accurately control the
polarization direction with respect to the optical axis of the liquid
crystal retarder 224a although the light is the linear polarization, a
polarizer 225 is disposed in the light pulse incidence surface side of
the liquid crystal retarder 224a. For example, the polarizer 225 is
disposed in the light pulse incidence surface side of the intensity
modulator 224, the position of the polarizer 225 is set so that the
optical axis of the liquid crystal retarder 224a is tilted by 45 degrees
with respect to the linear polarization, and the intensity modulator 224
and the polarizer 225 are integrated with the liquid crystal retarder
221. According to this configuration of the polarization modulating unit
22, although the position of the polarization modulating unit 22 is not
accurately controlled with respect to the light source unit 21, it is
possible to set the polarization direction and the optical axis of the
liquid crystal retarder 224a or the like at a desired angle.

[0188] The optical unit 31 of the reception side communication apparatus
30 includes a non-polarization beam splitter 311, polarization beam
splitters 312 and 315, and a 1/4 wavelength plate 313. In addition, the
reception side communication apparatus 30 includes an automatic
polarization controller 37 and a lens 319.

[0189] The non-polarization beam splitter 311 performs division without a
change in the polarization state of the light pulse emitted from the
transmission side communication apparatus 20. The polarization beam
splitter 312 performs the polarization separation on the one of the light
pulses split by the non-polarization beam splitter 311. With respect to
the polarization state of the other light pulse spilt by the
non-polarization beam splitter 311, the 1/4 wavelength plate 313 converts
the linear polarization to the circular polarization and converts the
circular polarization to the linear polarization. The polarization beam
splitter 315 performs the polarization separation on the light pulse of
which the polarization state is changed by the 1/4 wavelength plate 313.

[0190] The light-receiving unit 32 includes light-receiving devices 321H,
321V, 321R, and 321L. The light-receiving device 321H detects the one
light pulse which is polarization-split by the polarization beam splitter
312, and the light-receiving device 321V detects the other light pulse
which is polarization-split by the polarization beam splitter 312.
Similarly, the light-receiving device 321R detects the one light pulse
which is polarization-split by the polarization beam splitter 315, and
the light-receiving device 321L detects the other light pulse which is
polarization-split by the polarization beam splitter 315.

[0191] The automatic polarization controller 37 corrects the polarization
state of the light pulse transmitted through the optical fiber 52. The
lens 319 collimates the light pulse which passes through the automatic
polarization controller 37 to be emitted from the optical fiber 52 and
allows the light pulse to be incident on the non-polarization beam
splitter 311.

[0192] In addition, in the fifth embodiment, an optical system which
extracts the light pulse from the optical fiber 52 to a free space, after
the light pulse passes through the automatic polarization controller 37,
is illustrated. However, the optical system may be configured so that the
non-polarization beam splitter, the polarization beam splitter, or the
like is configured with optical fiber devices, so that the light pulse
may not be extracted to a free space but it may be divided by the optical
fiber.

6-2. Operations of Fifth Embodiment

[Quantum Communication Operation]

[0193] The controller 26 drives the semiconductor light-emitting device
211 of the light source unit 21 by the pulse current to generate the
light pulse. The light pulse generated by the light source unit 21 is
incident on the intensity modulator 224 of the polarization modulating
unit 22. In addition, in the case where the polarizer 225 is installed,
the light pulse is incident through the polarizer 225 on the intensity
modulator 224.

[0194] The liquid crystal retarder 224a of the intensity modulator 224 is
controlled at random so that the phase difference a occurring in the
polarization components along the FAST axis and the SLOW axis is one
value among a plurality of predetermined values according to the arrival
timing of the light pulse. in the case where the light emitted from the
light source unit 21 is the linear polarization, the polarization axis of
the polarizer 224b is set to be directed to the direction perpendicular
to the polarization direction; and in the case where the polarizer 225 is
installed, the polarization axis of the polarizer 224b is set to be
directed to the polarization axis of the polarizer 225. Therefore, the
phase difference a in the liquid crystal retarder 224a is controlled to
be a plurality of predetermined values by the controller 26, so that the
light pulse passing through the intensity modulator 224 is the light
pulse which the intensity modulation is performed on. The light pulse
passing through the intensity modulator 224 is incident on the liquid
crystal retarder 221. The liquid crystal retarder 221 performs the same
operations as those of the first embodiment, and the light pulse of which
the polarization state is controlled to be one of the four polarization
states at random by the controller 26 is emitted to the lens 226.

[0195] The lens 226 condenses the light pulse emitted from the liquid
crystal retarder 221 to be incident on the optical fiber 52 connected to
the optical fiber connector 27.

[0196] The reception side communication apparatus 30 allows the
non-polarization beam splitter 311 of the optical unit 31 to divide the
light pulse collimated by the lens 319. The one of the light pulses split
by the non-polarization beam splitter 311 is incident on the polarization
beam splitter 312, divided into the polarization components, and incident
on the light-receiving device 321H or the light-receiving device 321V.

[0197] The other of the light pulses split by the non-polarization beam
splitter 311 passes through the 1/4 wavelength plate 313 to allow the
polarization state to be changed, and after that, incident on the
polarization beam splitter 315, divided into the polarization components,
and incident on the light-receiving device 321R or the light-receiving
device 321L. By repetitively performing the above operations, the light
reception results of the light-receiving devices 321V, 321H, 321L, and
321R are output to the controller 36.

[0198] In addition, the transmission side communication apparatus 20 and
the reception side communication apparatus 30 perform operation setting
of the automatic polarization controller 37 so that a change in the
polarization state which inevitably occurs during the propagation of the
light pulse through the optical fiber 52 is removed before the quantum
encryption communication is performed. For example, at a predetermined
timing, the transmission side communication apparatus 20 transmits the
light pulse having a predetermined polarization state, and the reception
side communication apparatus 30 detects the light pulse with the
light-receiving device. The controller 36 controls the automatic
polarization controller 37 based on the detection result of the
light-receiving device so that the light pulse having a predetermined
polarization state is detected by a predetermined light-receiving device.
In other words, the automatic polarization controller 37 removes a change
in the polarization state which occurs during the propagation of the
light pulse.

[Classical Communication Operation]

[0199] After that, in the BB84 protocol combined with the decoy method,
the portion which performs the classical communication is executed.
Similarly to the fourth embodiment, the transmission side communication
apparatus 20 and the reception side communication apparatus 30 perform
(1) the basis exchange, (2) the estimation of the error rate, (3) the
error correction, and (4) the privacy amplification by using the public
communication line.

[0200] In addition, in the fifth embodiment, the decoy method may be
combined with the BB84 protocol using the polarization states of the four
types of linear polarizations or the 6-state protocol using the
polarization states of the four types of linear polarizations and the two
types of circular polarizations.

[0201] In this manner, in the fifth embodiment, the polarization
modulating unit 22 is configured with the liquid crystal retarder 221,
the intensity modulator 224, and the like, and the light pulse emitted
from the polarization modulating unit 22 is configured so as to be
incident on the optical fiber 52 through the lens 226. In this manner,
even in the case where the optical fiber is used as the quantum
communication line, since the quantum encryption communication apparatus
or system may be miniaturized, it is possible to mount it on a portable
apparatus. In addition, the optical fiber 52 is used as the quantum
communication line, so that the position alignment of the transmission
side communication apparatus and the reception side communication
apparatus is unnecessary, and it is possible to easily perform the
quantum encryption communication.

<7. State of Application to Electronic Apparatus>

[0202] Next, examples where the quantum encryption communication apparatus
is applied to electronic apparatuses are described with reference to
FIGS. 11 to 13. FIG. 11 illustrates an example of application to a mobile
phone.

[0203] In the case where the quantum encryption communication apparatus is
applied to the mobile phone, a semiconductor light-emitting device 211, a
lens 212, and a liquid crystal retarder 221 are mounted. The light pulse
emitted from the semiconductor light-emitting device 211 is condensed by
the lens 212 so as to be incident on the liquid crystal retarder 221.
Next, the liquid crystal retarder 221 performs the polarization
modulation of the incident light pulse and emits the light pulse to an
external portion of the mobile phone.

[0204] In this manner, the quantum encryption communication apparatus is
configured so the polarization modulation is performed by using the
liquid crystal retarder. Therefore, it is possible to miniaturize the
quantum encryption communication apparatus, and as illustrated in this
figure, it is possible to embed the quantum encryption communication
apparatus in the mobile phone.

[0205] FIGS. 12 and 13 illustrate examples of application to a notebook
type computer. In the case of the application to the notebook type
computer, for example, in the arrangement illustrated in FIG. 12, a
semiconductor light-emitting device 211, a lens 212, a liquid crystal
retarder 221, optical units 31 and 32, a filter 41, and an optical fiber
connector 42 are disposed. The configuration of these parts disposed in
the notebook type computer is the same as that of FIG. 13. The light
pulse emitted from the semiconductor light-emitting device 211 is focused
by the lens 212 so as to be incident on the liquid crystal retarder 221.
Next, the liquid crystal retarder 221 performs the polarization
modulation of the incident light pulse and allows, the
polarization-modulated light pulse to be incident on the filter 41.

[0206] The filter 41 functions as a light separator which guides the light
pulse output from the light source apparatus 1 to the optical fiber
connector 42 and guides the light pulse which is input from an external
portion through the optical fiber connector 42 to the optical unit 31.
Therefore, the polarization-modulated light pulse is incident through the
filter 41 on the optical fiber connector 42. The light pulse incident on
the optical fiber connector 42 is extracted to an external portion
through the optical cable connected to the optical fiber connector 42. On
the other hand, the light pulse which is incident from an external
portion through the optical cable connected to the optical fiber
connector 42 is incident through the filter 41 on the optical unit 31.
The optical unit 31 receives the incident light pulse. The optical unit
31 separates the received light pulse into the polarizations and outputs
the polarizations to the light-receiving unit 32 which is configured by
using a semiconductor light-receiving device, for example, a photodiode
or the like.

[0207] In addition, herein, the situation where bi-directional
communication is performed by using one optical cable has been
considered. In general, in the bi-directional communication using the
optical cable, the wavelengths of the light pulse used for the
transmission and the wavelengths of the light pulses used for the
reception are different from each other. Therefore, it is possible to
separate the light pulses of which the wavelengths are different by using
the aforementioned filter 41. According to a type or a form of
communication functions mounted on the notebook type computer, the
configurations of the filter 41, the optical fiber connector 42, the
optical unit 31, and the light-receiving unit 32 may be appropriately
modified. In addition, a partial change in design, for example, embedment
of the lens 212 in the light source apparatus or the like is also
permitted.

[0208] In this case, the quantum encryption communication apparatus is
configured to perform the polarization modulation by using the liquid
crystal retarder, so that it is possible to miniaturize the quantum
encryption communication apparatus and to embed the quantum encryption
communication apparatus in the notebook type computer as illustrated in
the figure. In addition, since the size of the notebook type computer is
larger than that of the mobile phone, it is possible to easily install
the reception function as well as the transmission function of the
quantum encryption communication. For example, as illustrated in the
figure, an optical filter is installed to pass the polarization-modulated
light pulse to a light connector socket side and to reflect the light
pulse of the light connector socket side to the optical unit 31 side.
According to this configuration, it is possible to install the
transmission function and the reception function of the quantum
encryption communication in the notebook type computer.

[0209] In addition, the present disclosure should not be construed to be
limited to the aforementioned embodiments of the present disclosure. The
embodiments of the present disclosure disclose the present disclosure in
exemplary forms, and thus, it is obvious that various modifications or
alterations of the embodiments may be made by those skilled in the art
within the range not departing from the spirit of the present disclosure.
In other words, the claims should be taken into consideration in order to
determine the spirit of the present disclosure.

[0210] The present disclosure contains subject matter related to that
disclosed in Japanese Priority Patent Application JP 2010-226628 filed in
the Japan Patent Office on Oct. 6, 2010, the entire contents of which are
hereby incorporated by reference.